Process control is a long-established art which plays a major role in managing industrial plants and processes. In this art, process transmitters have been used to monitor process variables. Having evolved from the earliest measurement devices such as barometers and thermometers, the process transmitter has traditionally received a great deal of technological attention to improve performance due to the need for accurate process measurement. Since the accuracy of every measurement made in a process control loop is directly dependent upon the accuracy of the particular process transmitter or instrument which closes the loop, the process transmitter plays a particularly sensitive role in industrial process control systems.
Beginning in the 1950s, electrical and electronic process control loops were a natural development from prior electro-mechanical control systems. The general problem of electronic process control is to convert a physical variable to an electrical signal, and to subsequently transmit that signal to a recorder and/or other control equipment which may be located some distance away from the physical variable. Early types of process control loops to accomplish this goal were "four-wire" systems, and were configured such that operating power was supplied through two of the four wires and a process signal was transmitted through the other two wires. The four-wire system requires the use of amplifiers or other signal conditioning equipment at the point of measurement in order to supply an accurate signal representative of the physical variable since the process signal is generally very low. See, e.g., U.S. Pat. No. 3,680,384, of Grindheim. Prior four-wire transmitter systems thus required separate power supply lines, and voltage power supplies.
After the four-wire transmitter was developed, it became apparent that the advantages of using the same two wires for power supply and information transmission would greatly improve the process control art. The "two-wire" transmitter was then developed and operates today in a control loop in conjunction with an external power supply, a pair of wires from the supply, and a transmitter connected serially between the wires. As used herein, the term "two-wire" is construed broadly to mean two conductors. Thus, the term "two-wire" includes actual wires, twisted pairs, coaxial cables, and other pairs of conductors.
During operation of such a two-wire transmitter loop, the transmitter energizes a sensor element and receives informational signals from the sensor element. The information is transmitted on the pair of wires by varying the current in the current loop. Thus the transmitter acts as a variable current sink, and the amount of current which it sinks is representative of the information from the sensor. Such prior two-wire transmitter loops have generally been analog in nature, and the industry standard which has developed for two-wire transmitters is a 4 to 20 milliamp loop, with a variable loop supply voltage having a maximum output of 24 volts DC. With such a low voltage supply, two-wire transmitter loops are particularly suited for use in hazardous environments. See, e.g., U.S. Pat. No. 4,242,665, of Mate.
In the operation of a typical process control system, a controller is usually located away from the source of the process variable which is to be measured and controlled. A controller may provide control of the process variable setpoint, and other parameters. Since prior controllers have usually been located both physically and electrically away from the process, they cannot be considered to be "integrated" with the transmitter in the loop.
Controllers provide an important function for process control systems, and remote communication units to provide control functions in a two-wire loop are known. See, e.g., U.S. Pat. No. 4,737,787, of Ito et al. The Ito et al. patent discloses data communication between a two-wire transmitter, and a receiver and communication unit which transmits digital data to a microprocessor in the transmitter. See col. 2, line 46 through col. 3, line 12 of the Ito et al. patent. The communication unit provides control to the entire system by operator command. See col. 3, lines 19-22 of the Ito et al. patent. The device disclosed in the Ito et al. patent is adaptable for future digital control and can be remotely adjusted by altering the device's communication state. See col. 7, lines 58-65 of the Ito et al. patent.
More advanced prior two-wire transmitter control loops exhibit high-level data communication between two-wire transmitters and various receiving elements, for example controllers and communication devices, For example, the concept of digital communication in 4 to 20 milliamp control systems is known for use in the more complicated 4 to 20 milliamp loops having both digital and analog components. Transmitters suitable for such purposes are usually called "smart" transmitters because they are more accurate and have operating parameters which may be remotely controlled.
As technology has progressed over the years, low powered microprocessors have made it possible to transport smart field transmitters into the digital signal processing environment. Furthermore, digital microprocessors make it possible to improve the accuracy of smart two-wire transmitters while allowing a communications protocol to be added to the loop.
Modern digital transmitters are also available with both hybrid and pure digital outputs. Thus, hybrid transmitters provide a 4 to 20 milliamp output signal along with a simultaneous digital communication signal to efficiently interface with a standard 4 to 20 milliamp analog loop. Transmitters with purely digital outputs provide communications on a dedicated data highway which is a data bus that carries the digital signal. On the highway, high level devices may be located which are capable of decoding the information received from the transmitter to aid in controlling the process.
Generally, to obtain higher accuracy smart transmitters, microprocessor-based units use digitally stored data in a local memory to provide precise corrections for the non-linearities associated with individual sensors in the loop. Smart transmitters can thus store, for example, ranging data indicative of the upper and lower values exhibited by the process variable, and can be reranged by instructing the microprocessor to look up and use a different set of range values. Smart transmitters also offer the future capability of fully digital communication with the control room.
Other smart transmitters utilize particular digital protocols for bidirectional communication between microprocessors and a loop interface, for example. See, e.g., U.S. Pat. No. 4,796,256 of Opderbeck et al. The Opderbeck et al. patent discloses a mini-packet receiver transmitter (MPRT) which provides an interface between one or two 8-bit microprocessors and a digital subscriber loop interface. See col. 2, lines 36-40 of the Opderbeck et al. patent. A "ping pong" protocol is bussed on a twisted pair to provide bidirectional communication between the microprocessors and the loop interface. See col. 2, lines 40-49 of the Opderbeck et al. patent. The ping pong protocol controls alternation of the receive and transmit mini-packet frames in the MPRT for implementation of the frame format. See col. 3, lines 26-28.
Another example of a smart, modular two-wire industrial control transmitter having connectable modules for use in a loop is shown in U.S. Pat. No. 4,818,914, of Orth et al. The Orth et al. patent teaches a two-wire industrial transmitter having a modular construction comprising a detector module connected by a serial bus to an output module. The output module includes a microprocessor and circuitry to output a sensed parameter. The detector module may include a plurality of sensing means to produce digital signals, at least one of which is used for correcting the digital signal representing the process variable. See col. 1, lines 32-49. The Orth et al. patent teaches a system having possible multi-drop mode configuration and coordination between sensors, for example, temperature and pressure transducers.
The trend in two-wire transmitter loops both in the smart, microprocessor-based transmitter area and the traditional analog transmitter area, has been to reduce the power requirements for components which are used in the loop. This need has arisen since the amount of power which a two-wire transmitter may draw from a current loop to use for its operation is severely limited. With a nominal 10-volt supply, at the bottom end of operation only about 40 milliwatts is available to power any instrumentation in the loop. Thus with large power demands on the loop, two-wire control systems may be limited to a few low power industrial control applications. This aspect of industrial controls competes with the general desire to design instrumentation into the loop to simplify loop operation and installation, and to provide intrinsic safety in a low power process control environment. This long-felt need has not adequately been met by process control loops which have the aforementioned inherent power budget problems.
While the aforementioned two-wire process transmitters and systems have been useful for many purposes, it has remained important to improve their accuracy and system response, to make them more reliable and to make them less expensive. It is also important to make process transmitters and controllers easier to install in a process loop and to improve their power consumption efficiencies, while also enabling use of plural transmitters and controllers to operate on a field bus or on the same two wires. Furthermore, it is desirable to provide the ability for bi-directional communications over the loop.